The present invention relates to optical scanning devices and image forming devices, and in particular to techniques for suppressing warping of housings in optical scanning devices.
Among electrophotographic image forming devices are image forming devices that use an optical scanning device to form an electrostatic latent image through exposure of a photoreceptor. An optical scanning device scans a photoreceptor surface with a laser beam by deflecting the laser beam emitted from a laser diode (LD), by using a deflecting mirror and an optical element housed in a housing. The deflecting mirror is held by a deflector, and the deflector is generally positioned near a center of the housing.
The deflector drives the deflecting mirror to rotate during exposure (scanning), but the center of gravity of the deflecting mirror is not on the rotation axis, and therefore the deflecting mirror vibrates due to the rotation. When the vibration of the deflecting mirror is transmitted to the optical element via the deflector and the housing, and these members vibrate, beam performance on the photoreceptor (image plane) changes.
The housing has a lowest rigidity around the center of the housing, and therefore when the deflector positioned near the center of the housing vibrates, the vibration tends to become large. Further, in order to increase a number of images formed per unit time, or increase resolution, it becomes necessary to increase rotation speed of the deflecting mirror. However, when the rotation speed of the deflecting mirror is increased, vibration of the deflecting mirror becomes larger. Thus, if these contributing factors overlap, there is a risk of image deterioration due to a change in beam performance.
In response to such circumstances, for example, countermeasures have been proposed such as increasing rigidity in the vicinity of the center of the housing (Japanese Patent Application Publication 2013-186335). More specifically, as illustrated in FIG. 12, in a housing 12 that has a rectangular bottom plate 1201 and a side wall 1202 standing upright from the periphery of the bottom plate 1201, ribs 1203 stand upright in the vicinity of a central portion of the bottom plate 1201, and ends of the ribs 1203 connect with the side wall 1202.
This increases rigidity of a central portion of the housing 12, and therefore even when vibration of the deflecting mirror 1204 is transmitted to the bottom plate 1201, vibration of the bottom plate 1201 is restricted by the ribs 1203. Accordingly, vibration energy transmitted from the bottom plate 1201 to the optical element 1205 and the like is suppressed, suppressing vibration of the optical element, and therefore it is possible to prevent image deterioration.
The housing 12 is formed by casting a high temperature mold material in molds 1301, 1302, as illustrated in FIG. 13A. Heat of the mold material is conducted to the molds 1301, 1302, and further radiated from the molds 1301, 1302 to surrounding space. Thus, peripheral portions of the molds 1301, 1302 become low temperature regions.
Further, among spaces surrounded by the bottom plate 1201 and the side wall 1202, spaces 1311, 1312 partitioned by the ribs 1203 both have low temperature regions of the molds 1301, 1302 above, below, and on the other side of the side wall 1202, and therefore the spaces 1311, 1312 become mid-range temperature regions of the mold 1301. A space 1313 sandwiched between the ribs 1203 has low temperature regions of the molds 1301, 1302 above and below, but is sandwiched between the spaces 1311, 1312, which mid-range temperature regions, making dissipation of heat difficult, and therefore the space 1313 becomes a high temperature region of the mold 1301.
As illustrated in FIG. 13B, in a process in which a mold material of a housing 1330 is solidified by cooling from a high temperature state, if surface temperatures of molds 1321, 1322 in contact with two main surfaces of a bottom plate 1331 are different, a shrinking force is greater on a high temperature side of the bottom plate 1331 than on a low temperature side. As a result, a difference in shrinkage occurs between two primary faces of the bottom plate 1331, meaning the bottom plate 1331 warps after casting, bending out towards what was the low temperature side.
When the bottom plate 1331 of the housing 1330 warps, accuracy of dimensions of the housing 1330 cannot be ensured, and therefore misalignment can occur between the deflector and the optical element, and accurate beam performance on an image plane cannot be ensured.